Tonight I have two mortise gauges to look at. I bought one at Ed Lebetkin’s tool store above Roy Underhill’s Woodwright’s School. Chris Schwarz wrote about Ed’s stuff here: http://lostartpress.wordpress.com/2011/07/22/sometimes-temptation-comes-from-above/
I managed to get out of there with only this gauge. Just a few weeks ago, Jennie Alexander & I were talking about this model of gauge, and how it strikes a pair of lines that are staggered where they begin & end. It makes sense, and if we were to find a pair of lines like that on old furniture, we could say the joiner likely used a gauge like this one. (or a marking gauge struck twice) Well, in principal anyway.
The wear on this one, either on the beams that the pins are in, or on the pins themselves result in one pin striking a stronger line than its neighbor. Thus you need to go over the lines a bit, and that gets you a varied pattern of struck lines. they aren’t always consistent. So much for theory. But, it’s a nice mortise gauge. The beams have a small tongue & groove to keep them together, and the whole thing tightens with a wedge thru the head. The plan is to one day make one based on this…but that was the plan when I bought this next mortise gauge umpteen years ago.
This little one is more complicated than the previous one. It has two beams that nest together, and are captured in the head by a wedge. So far, so good. But one of the pins is installed in a block glued onto one beam. Then this block fits in a square notch cut in the other beam. Nice.
As an added registration, there’s a groove in one beam, and a small iron pin in the other to keep them keyed to each other.
Someone marked positions on the edges of the beams to line up the pins for specific mortises. Here’s the main setting – 3/8″
Shift things this way & you have 5/16”
and this way gets you 7/16”
all with two marks scribed across the beams. nice one.
But remember, these are more complicated than they need to be – here’s Alexander’s fixed gauge. one setting, over & over. https://pfollansbee.wordpress.com/2011/07/19/a-simple-mortise-gauge-and-some-mail/